This invention relates to polyvinyl chloride based resin compositions having such an excellent ability to absorb vibrational energy that they are suitable for use in various applications including transportation equipment, precision electronic devices and acoustic devices. Through effective control of vibrations, the compositions are capable of improving the response speed of transportation equipment, increasing the precision of measurement with electronic devices and enhancing the sound quality and comfort of acoustic devices.
Butyl rubbers have been used most extensively as materials to absorb vibrational energy. Recently, polynorbornene rubbers and specialty urethane based elastomers have been found to have better performance and drawn the attention of researchers. The evaluation of these material for absorbing vibrational energy is primarily performed in terms of the storage modulus (E') and loss tangent [tan .delta.= loss modulus (E")/storage modulus (E')] that are determined by measuring their viscoelastic properties.
For designing successful vibration absorbing materials, their loss tangent is preferably as high possible whereas the storage modulus has an optimum value depending on the form in which they are used. These two factors usually have high dependency on temperatures. The storage modulus decreases gradually with increasing temperature and decreases sharply at certain temperatures, typically beyond the glass transition point. On the other hand, the loss tangent shows the highest value at temperatures slightly beyond the glass transition point and tends to decrease at lower or much higher temperatures.
Under the circumstances, the first criterion for successful vibration absorbing materials has been that they have high loss tangents in the temperature range in which they are used. As for the storage modulus, it has been possible to attain an optimum value since adjustments can be effected over a considerably broad range by addition of inorganic or metallic fillers, softening agents, rubber, etc. As a result, butyl rubbers, polynorbornene rubbers and specialty urethane based elastomers have attained excellent loss tangents (tan .delta.) that are respectively 1.4, 2.8 and 1.3 at maximum. However, these materials have found only limited use because of their relatively low processability and formability.
The recent demands for higher performance and quality of precision electronic devices and various transportation equipment including automobiles are growing every year and it is required today that the value of loss tangent (tan .delta.) be high not only in a specified temperature range but also in a broad temperature range of from room temperature to near 60.degree. C., or even from -20.degree. C. to near 100.degree. C. depending on use.
Polyvinyl chloride resins have long been used as a class of general-purpose resins and not only their economy but also practically all of the methods of shaping and processing them have been established. As other advantages, they are noncrystalline resins and permit easy production of composites with inorganic or metallic fillers or softening agents.
Polyvinyl chloride alone has a loss tangent that peaks at about 1.1 at temperatures of ca. 90.degree. C. If di-2-ethylhexyl phthalate (hereinafter abbreviated as DOP) which is a typical plasticizer is added in an amount of 100 parts by weight per 100 parts by weight of the resin, the peak temperature of the resin's loss tangent will drop to ca. 5.degree. C. and at the same time the peak value of the loss tangent also drops to ca. 0.7. It has heretofore been held that this phenomenon is due to the broadening of the distribution of relaxation time that occurs as a result of the entrance of a dissimilar molecule into the moleculecular chain of polyvinyl chloride per se. However, the recent studies of the present inventors have revealed that when a very limited class of plasticizers typified by dicyclohexyl phthalate are added to polyvinyl chloride, the peak temperature of its loss tangent decreases and yet the peak value of loss tangent rises to ca. 1.6. A fatal problem with this approach is that bleeding occurs if more than 70 wt. % of dicyclohexyl phthalate is added independently to polyvinyl chloride or even if it is added in an amount of less than 30 wt. % an admixture with DOP.
Foams are extensively used as abosrbents, particularly for absorbing vibrational impacts and polyvinyl chloride paste resins are used most commonly in this area of applications. However, polyvinyl chloride paste resins are typically used as sol after being mixed with liquid plasticizers and hence are not highly compatible with dicyclohexyl phthalate which is solid at room temperature.